Every building must have an entrance. Entrances are often used to create the principle
vision of the architectural outcome. Windows are needed for light and ventilation.
In conjunction with the other building elements (roofs & walls) the windows and doors
are often used to define the architecture, from a design and historical point of
view. Windows give the visual link to the outside and as such are designed with
this in mind. As with the other elements, regulations often require that the thermal
performance of the windows and doors are allowed for in the heat loss calculations.
Other issues that may also need to be considered are heat gain, overlooking, wind
loading, fire performance, etc.